1. Field of the Invention
The present invention relates to a semiconductor memory device and a method of fabricating the same and, more particularly, to an MRAM (Magnetic Random Access Memory) using a TMR (Tunneling Magneto Resistive) element as a memory element and a method of fabricating the same.
2. Description of the Related Art
Recently, an MRAM (Magnetic Random Access Memory) cell using the tunnel magneto resistive (to be referred to as TMR hereinafter) effect as a data memory element has been proposed.
FIG. 14 is a plan view of a semiconductor memory device according to prior art. FIG. 15 is a sectional view of this semiconductor memory device taken along a line XV—XV in FIG. 14. FIG. 16 shows the magnetization state in a magnetic recording layer of the semiconductor memory device according to this prior art.
As shown in FIGS. 14 and 15, bit lines 11 and write word lines 13 are so formed as to cross each other at right angles. TMR elements 23 are formed at the intersections of the bit lines 11 and the write word lines 13. One end of each TMR element 23 is connected to the bit line 11, and the other end of the TMR element 23 is connected to a read word line 30 via a lower electrode 40 and a contact 41.
The TMR element 23 has a three-layered structure including two magnetic layers and a nonmagnetic layer sandwiched between these magnetic layers. That is, the TMR element 23 is composed of a magnetic recording layer 24 which connects to the bit line 11 via an upper electrode (not shown), a magnetization fixing layer 25 which connects to the lower electrode 40, and a thin tunnel insulating film 19 sandwiched between the magnetic recording layer 24 and the magnetization fixing layer 25.
This semiconductor memory device according to the prior art has the following problems.
First, the magnetic recording layer 24, the magnetization fixing layer 25, and the tunnel insulating film 19 constructing the TMR element 23 are formed in a plane parallel to a semiconductor substrate (not shown) on which this TMR element is mounted. When the TMR element 23 is patterned, therefore, the surface area of this TMR element 23 depends upon the minimum dimension of lithography. That is, the degree of freedom of processing of the TMR element 23 is low.
Also, it is originally ideal in the magnetic recording layer 24 that all the magnetization directions point in the same direction. In practice, however, as shown in FIG. 16, a magnetic domain 100 in which the magnetization vectors in the longitudinal direction turn is generated in each of the two end portions of the magnetic recording layer 24. This magnetic domain 100 generates a so-called demagnetizing field. Consequently, a region in which this demagnetizing field is generated can no longer uniformly maintain tunnel resistances corresponding to the original storage states of data “1” and “0”. This problem becomes conspicuous when the TMR element 23 is downsized. That is, to downsize the area component, which is parallel to the semiconductor substrate, of the TMR element 23, the surface area of the TMR element 23 must be decreased. More specifically, as the surface area of the TMR element 23 decreases, the proportion of the magnetic field unstable region generated by the magnetic domain 100 in the end portion of the TMR element 23 increases. This makes it difficult to detect a difference in change amount between the tunnel resistances. Additionally, downsizing of the film thickness component, which is perpendicular to the semiconductor substrate, of the TMR element 23 is more difficult than downsizing of the area component of the TMR element 23. Therefore, if downsizing of the area component of the TMR element 23 is advanced, a magnetic field required for switching increases, and this extremely increases an applied current when the magnetic field is generated. As described above, since downsizing of the TMR element 23 is difficult, downsizing of a cell is also difficult.
Furthermore, as shown in FIG. 15, the conventional cell requires one bit line 11 and two word lines (the write word line 13 and the read word line 30) for each TMR element 23. In addition, to connect the TMR element 23 to the read word line 30, wiring must be extracted by using the lower electrode 40, the contact 41, and the like. Accordingly, the existence of various wiring and the like increases the minimum processing dimension of the cell to 8 F2 or more (FIG. 14). This further makes downsizing of the cell difficult.
Also, as shown in FIG. 15, as a distance X′ between the write word line 13 and the TMR element 23 is shortened, the write current decreases, and this improves the operation margin. Hence, it is necessary to shorten this distance X′ between the write word line 13 and the TMR element 23. However, it is very difficult in view of process to perform control such that a film thickness 16a of the insulating film between the write word line 13 and the TMR element 23 decreases.
As described above, in the semiconductor memory device according to the above prior art, the degree of freedom of the surface processing of the TMR element 23 is low, downsizing of the cell area is difficult, and control of the distance X′ between the write word line 13 and the TMR element 23 is difficult.